There are a multitude of sensor elements and methods for detecting at least one property of a measured gas in a measured gas chamber in the related art. In this context, it may be, in general, any physical and/or chemical property, one or more properties being able to be measured. In the following, the present invention is described, in particular, with reference to a qualitative and/or quantitative determination of a level of a gas component of the measured gas, in particular, with reference to a determination of an oxygen concentration in the measured gas. The oxygen concentration may be measured, for example, in the form of a partial pressure and/or in the form of a percentage. However, alternatively or in addition, other properties of the measured gas, such as the temperature, are also measurable.
For example, such sensor elements may take the form of so-called oxygen sensors, as are known from, e.g., Konrad Reif (ed.): Sensors in the Motor Vehicle, 1st ed., 2010, pages 160-165. Using broadband oxygen sensors, in particular, planar broadband oxygen sensors, e.g., the oxygen concentration in the exhaust gas may be determined in a large range, and therefore, the air-fuel ratio in the combustion chamber may be inferred. The excess-air factor describes this air-fuel ratio.
The related art describes, in particular, ceramic sensor elements, which are based on the use of electrolytic properties of specific solids, and thus, on ion-conducting properties of these solids. In particular, these solids may be ceramic solid electrolytes, such as zirconium dioxide (ZrO2), especially, yttrium-stabilized zirconium dioxide (YSZ) and scandium-doped zirconium dioxide (ScSZ), which may include small additions of aluminum oxide (Al2O3) and/or silicon dioxide (SiO2). Thus, in the case of such sensor elements, e.g., a laminated construction of ceramic substrates having inner heating-element and electrode structures is known, which are connected to a voltage source and/or evaluation unit via a plated-through hole.
In spite of the numerous advantages of the conventional sensor elements, they still have potential for improvement. Increasing functional demands are being placed on such sensor elements. Different and sometimes mutually opposing requirements must be satisfied for a robust and reliable design of the plated-through holes. Thus, there must be a high electrical resistance between the plated-through holes, even at operating temperatures above 500° C. This may realized, for example, by an insulating layer of electrically insulating material, such as aluminum oxide. The basic mechanical strength of the YSZ carrier body ceramic may only be reduced slightly by the introduction of several plated-through holes. This may be achieved by small diameters and smooth, crack-free walls of the plated-through holes. The plated-through holes should be produced by stable and inexpensive manufacturing processes usable in the case of multiple printed circuits. This may be achieved, for example, using multiple stamping, boring or stack suction. In addition, breakup of the YSZ carrier ceramic by hydrothermal aging over the lifetime of the sensor element must be prevented. Hydrothermal aging involves, in particular, a phase transition of the zirconium dioxide from the tetragonal phase to the monoclinic phase, which may produce a mechanical breakup or even a rupture of the weakened ceramic in response to vibration or shocks. The phase transition may begin, in particular, in the region of the stamping burr, thin through-hole plating, a niobium-free terminal contact paste, and missing insulation.